The invention concerns a vacuum switching device, and in particular a vacuum interrupter.
Vacuum interrupters are commonly used in electrical equipment for interrupting an AC supply in the event of a fault, e.g. a short-circuit on a power line. A typical vacuum interrupter is shown in very general terms in FIG. 1. The interrupter comprises an insulator 10, normally made of a ceramic or glass material, housing two electrically conductive contacts 11, 12. Contacts 11, 12 are taken out of the interrupter unit by means of respective stems 13, 14, the stems terminating in end-portions 15, 16, normally referred to as "end-stubs", for connection to further electrical equipment (not shown). The end-stubs 15, 16 may have external or internal threads for effecting the connections. The interrupter by means of its contacts serves selectively to establish or remove electrical continuity between the further electrical equipment and the AC supply.
Also included in the interrupter is a bellows unit 17 and a shield 18. The bellows unit 17 allows axial movement of the stem 14 to make and break, as afore-mentioned, electrical contact between the contacts 11 and 12, contact 11 and stem 13 being fixed relative to the insulator 10.
The shield 18 is an electrically conductive component which serves two main purposes: to prevent an arc, which is drawn when the contacts are separated, from striking the insulator and to impede the deposition of metal vapour, which is given off from the contacts when the arc is present, on the insulator.
The arc that is drawn when the contacts are separated during the presence of a normal or a high fault-current, for example, allows the current flowing prior to the interruption to continue by the medium of metal vapour given off from the contact faces. Nominally the arc would extinguish when the current passing through the arc passed through its next zero-crossing, however a phenomenon known as "current chopping" causes the arc to cease ("chop") before that zero-crossing point by virtue of the reduction of the energy in the arc. When chopping occurs, a high voltage can be caused to appear across the contacts which is passed on to equipment (e.g. a motor load) connected to the interrupter, and if the voltage is high enough damage can be done to that equipment. There is therefore a desire to keep such voltage to as low a level as possible, which in turn means minimizing the current at which chopping occurs.
The value of the chopping current depends on the nature of the contact material and it has been found that, although an element such as copper or silver by itself gives rise to a high level of chopping current, if such a high-conductivity material is combined with an arc-resistant material such as tungsten, tungsten carbide or chromium, the chopping current can be brought down to very low levels, e.g. of the order of 4 A or less.
Use of a combination of materials for the contact instead of just the high-conductivity material considerably increases the costs of the interrupter and in an attempt to minimize such costs it is common practice to make only part of each contact of a combination of materials the remaining part being of the high-conductivity element only. The combined-material part is that part from which the arc is struck, the remaining part of each contact serving to sink heat from the arc-exposed part and to physically and electrically connect that part to the contact stem. It is clear that, since this remaining part plays no role in arc generation, it is not required to supply a vapour which has the afore-mentioned low chopping current quality, and can therefore be made exclusively of inexpensive high-conductivity material.
It is an aim of the present invention to provide a vacuum switching device which permits increased savings in contact-material outlay.